The field of the invention is that of evaluating power levels in a cell of a mobile radio network, in particular to determine the load of that cell and to apply a call admission control procedure.
A mobile radio network includes base stations each of which serves mobile stations, known as active users, located in a cell associated with the base station. The signal transmitted by a mobile station to the base station is received by the station at a particular power level. Maintaining the total power level received from all active users within certain limits is essential to achieving good network performance by limiting the level of interference; the transmitted power level in one cell is perceived as interference in adjoining cells.
This is particularly true in a CDMA network, in which constant modification of the power transmitted by the mobile stations as a function of external conditions is critical for network efficiency. One way of maintaining the total received power level at the base station within certain limits is to verify if the additional connection is likely to increase the total received power level beyond the limit in the event of a request to connect an additional user, referred to as a candidate user, to a base station. The connection is granted only if the limit is not exceeded. This verification is based on evaluating the impact of connecting the candidate user on the total received power level at the base station.
The total received power level at the base station is the sum of three contributions. A first of these represents the individual power levels received from each of the users, a second represents an intercellular interference level, and a third represents a thermal noise level. The intercellular interference level corresponds to all of the individual received power levels at the base station concerned from all active users outside the cell associated with the base station concerned.
In the prior art, the impact of admitting a candidate user is determined by assuming that the base station receives a power contribution Prxtarget which is exactly the same from each user actually connected, referred to as an active user. This is in fact the ideal case that generates minimum interference between the various active users served by the base station. Admitting a candidate user is assumed to make an additional power contribution Prxtarget identical to that of the active users and added thereto. The term Prxtarget represents the received power level from each user at the base station. It is generally weighted by a coefficient associated with a user as a function of the type of service carried by the connection. This coefficient depends in particular on the activity on the line. Accordingly, the relationship between the received power level {tilde over (P)}rxtot assuming that a candidate user is admitted and the power level Prxtot actually received from active users is of the form {tilde over (P)}rxtot=Prxtot+xcex1N+1Prxtarget, where xcex1N+1 is a coefficient associated with the candidate user as a function of the type of service required, the candidate user being added to N active users in the cell.
The drawback of the above method is that the assumption about admission of the user candidate takes no account of the modifications to the system that it causes.
Firstly, adding a further user represents a source of interference for the active users of the cell. To maintain exactly the same receive quality at the base station, each active user must increase its transmitted power level.
Secondly, adding a user increases the total power received by the base station serving the additional user. This increases the level of intercellular interference received by neighboring base stations. Using a power control mechanism, the neighboring stations cause the mobile stations they are serving to increase their transmitted power level in order to maintain a satisfactory signal-to-noise ratio. This increase in the power transmitted by active users served by neighboring base stations leads in turn to an increase in the interference level at the base station which admitted the candidate user. The base station requests all active users it is serving to increase their transmitted power level to maintain a satisfactory signal-to-noise ratio.
Allowing for the admission of the candidate user, and by virtue of the two effects described above, the individual power level that will be received at the base station from each active user will therefore be greater than the individual received power level that would be received at the base station from each active user if an additional user were not admitted.
One particular object of the present invention is to specify the relationship between the total received power levels at the base station before and after admitting an additional user.
Another object of the invention is to use that relationship to evaluate the load of a cell precisely.
The above objects, and others that become apparent hereinafter, are achieved by a method of predicting the level of power received at a base station assuming a candidate user is admitted and allowing for an increase in the individual power levels of all active users already being served by the base station. This method therefore takes account of the two effects described above.
The relationship between the received power level {tilde over (P)}rxtot assuming that a candidate user is admitted and the power level Prxtot actually received from active users is preferably of the form {tilde over (P)}rxtot=xcex1.Prxtot where xcex1 is a factor greater than 1 depending on the number of users already connected, on coefficients characteristic of the type of service used by each active user and on a coefficient characteristic of the type of service required by the candidate user.
The above relationship advantageously takes into account the increase in intercellular interference due to the admission of an additional user. The relationship is then of the form {tilde over (P)}rxtot=xcex1.Prxtot+xcex2, where xcex2 depends on the value of the intercellular interference before admitting a candidate user and also on the same parameters on which xcex1 depends.
In another embodiment of the invention, the relationship is used to predict the load of the cell corresponding to a base station if the base station admits an additional user. In this way the estimated value of the load of the cell indicates if admission is possible without exceeding the load limits of the cell. This method is preferably used in a base station to apply a call admission control procedure in order to avoid overloading the system.